(1) Field of the Invention
The present invention relates to a process for producing ceramic honeycomb structural bodies. More particularly, the invention relates to the ceramic honeycomb structural body-producing process including an improved firing step.
(2) Related Art Statement
Heretofore, it has been common practice to fire extruded green ceramic honeycomb structural bodies by passing the honeycomb structural bodies through a tunnel kiln, wherein the honeycomb bodies are placed on a shelf plate of a truck, while cell-opened end surfaces contact the shelf plate. In order to improve this firing process, examined Japanese patent publication No. 1-54,636 discloses a process for firing green ceramic honeycomb structural bodies 21 in the state that a setter 23 is placed between the honeycomb structural body 21 and a shelf plate 22 as shown in FIG. 4. This setter is a ceramic plate having a honeycomb structure with an upper edge being chamfered. Japanese patent application Laid-open No. 2-199,067 discloses a process for firing green ceramic honeycomb structural bodies in the state that an alumina base solid plate is placed between the honeycomb structural body and the shelf plate.
However, the process using the setters as disclosed in examined Japanese patent publication No. 1-54,636 has the following problems:
(i) With increase in the number of times of passing the setter through the tunnel kiln, the setter is largely deformed (warped) by heat, or the surface of the setter becomes coarse due to fusing-out of the silica contained in the setter. As a result, the lower end face of the honeycomb structural body is deformed, or ribs are deformed or cracked.
(ii) The service life of the setter is short, i.e., the number of times of passing the setter through the tunnel kiln is as small as 35 to 40 due to the above problems.
(iii) Difference in dimension between upper and lower portions of the product is great due to difference in temperature between them.
On the other hand, the process using the alumina base plate as disclosed in the above unexamined Japanese patent application Laid-open No. 2-199,067 has the following problems:
(i) With increase in the number of times of passing the alumina base plate through the tunnel kiln, the base plate is warped or deformed due to the thermal deformation of the shelf plate, when the alumina base plate is thin. As a result, the lower end face of the honeycomb structural body is deformed or cracked.
(ii) When the alumina base plate is thick, the weight per unit area is increased which increases heat capacity.
(iii) The lower end face of the honeycomb structural body changes color by reacting with the alumina base plate. It is believed that the lower end face of the honeycomb structural body changes color since the composition of cordierite constituting the honeycomb structural body at the lower end face slightly changes due to the reaction between the alumina base plate and the cordierite during firing.